EDUCATIONAL ACTIVITY. Measuring local atmospheric changes during the

solar eclipse (2012 Total Solar Eclipse).

Authors:○ Mr. Miguel Ángel Pío Jiménez. Astronomer of the Institute of Astrophysics of

Canary Islands.○ Dr. Miquel Serra-Ricart. Astronomer of the Institute of Astrophysics of Canary

Islands.○ Sr. Juan Carlos Casado. Astrophotographer of tierrayestrellas.com, Barcelona.○ Dr. Lorraine Hanlon. Astronomer University College Dublin, Irland.○ Dr. Luciano Nicastro. Astronomer Istituto Nazionale di Astrofisica, IASF Bologna.○ Dr. Davide Ricci. Astronomer Istituto Nazionale di Astrofisica, IASF Bologna.

Collaborators:

○ Dr. Eliana Palazzi. Astronomer Istituto Nazionale di Astrofisica, IASF Bologna.○ Mrs. Emer O Boyle. University College Dublin, Irland.

1. Activity goals

In this activity we will observe atmospheric changes, especially changes in temperature due to the decrease in solar radiation that is caused by the blocking of the disk of the Sun by the Moon during a total solar eclipse. For this, a weather station located in Australia will be used.

The objectives are:

○ To understand the basic phenomenology of eclipses.○ To apply mathematics (algebra) and basic physics (thermodynamics) to calculate

temperature and radiation measurements from a weather station.○ To develop an understanding and to apply basic statistical analysis techniques (error

calculation).○ To work cooperatively, as part of a team, valuing individual contributions.

2. Instrumentation

A weather station that has sensors for measuring temperature and solar radiation intensity will be used for this activity. The students will have access to live data or to a database of archived data at a later time. A web-tool will be available to allow students to do the activity.

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3. Phenomenon

3.1 What is an eclipse?

An eclipse is the temporarily obscuration of a celestial body caused by the interposition of another body between this body and the source of illumination. In the following we will consider eclipses occurring in the Sun-Earth-Moon system where the term eclipse applies to two very different phenomena:

1. A solar eclipse occurs when the Moon passes between the Sun and Earth, and the Moon fully or partially blocks the Sun. This can happen only at New Moon (Moon between the Sun and Earth) and if the Sun and the Moon are perfectly aligned as seen from Earth. In a total eclipse, the disk of the Sun is fully obscured by the Moon. In partial and annular eclipses only part of the Sun is obscured.

2. A lunar eclipse occurs when the Moon passes directly into the Earth’s shadow. This can occur only when the Sun, Earth, and Moon are aligned exactly, or very closely, with the Earth in the middle. Hence, a lunar eclipse can only occur the night of a Full Moon.

3.2 Conditions for eclipses to occur

We know that the orbits of the Earth and the Moon are not coplanar, so most of the time the

Moon is above or below the plane of the ecliptic (the plane defined by the Earth's orbit around the Sun). For an eclipse to occur, the Moon has to be in, or very close to, the plane of the ecliptic and in New Moon phase (solar eclipse) or Full Moon (lunar eclipse).

Figure 1: The ecliptic plane and the Moon's orbit. The "critical zone" indicates a strip in which an eclipse may occur. (Diagram starryearth.com). The ‘Moon node line’ refers to the line joining the centre of the Earth to the point where the Moon’s orbit intersects the ecliptic plane.

The conditions in which solar eclipses can occur, happen two or three times a year -

every 173.31 days - in the so called eclipse seasons. The eclipse year is the time between two

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alignments of Sun, Moon and Earth and it is 346.62 days. During this time two eclipse seasons happen.

The Moon’s orbital node lines don’t have a fixed orientation, but rotate by about 20° per year,

going through one full turn in 18.6 years. This means that the dates on which the eclipses happen change each year. For example the eclipses of 2001 were in the months of January-February, June-July and December, the eclipses of 2003 were in May and November and those of 2006 in March and September. The movement of the orbital nodes means that the eclipses occur throughout the ecliptic.

Figure 2: Diagram of the umbral and penumbral zones in an eclipse.

3.3 Number of eclipses per yearThe minimum number of eclipses that occur each year is four - two solar eclipses and two

lunar eclipses. The maximum possible number of eclipses per year is seven, and only rarely happens. The

following combinations are possible: ● 5 solar eclipses and 2 lunar eclipses● 5 lunar eclipses and 2 solar eclipses● 4 solar eclipses and 3 lunar eclipses● 4 lunar eclipses and 3 solar eclipses

3.4 Types of solar eclipseThere are different types of solar eclipse, depending primarily on the length of the Moon’s

shadow and the Moon’s distance from Earth. These are illustrated in Figure 3.

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1) Partial eclipse: Only the Moon’s penumbral shadow reaches the Earth's surface (see

Figure 3, position C). These eclipses always occur at high latitudes (north or south).2) Annular eclipse: The Moon is too far from Earth for its umbral shadow to completely

cover the disk of the Sun, but it still blocks out most of the Sun light, leaving only a ring of light visible (Figure 3, position B).

3) Total eclipse: In this case the Moon is sufficiently close to Earth for its penumbral shadow to reach Earth, perfectly blocking the complete disk of the Sun (Figure 3, position A).

It is worth pointing out that solar eclipses are seen on Earth only because of the happy coincidence that, at some times during the year, the angular sizes of the Moon and Sun are identical. Hundreds of millions of years in the past, the Moon was too close to the Earth to precisely cover the Sun as we can now observe. Tidal forces cause the orbit of the Moon around Earth to increase by about 3.8 cm each year and in just under 1.4 billion years, the distance from Earth to the Moon will have increased by 23,500 km. After that, the Moon will no longer completely cover the Sun's disk as seen from Earth. Therefore, the last total solar eclipse on Earth will occur in about 1.4 billion years from now!

Figure 3: Diagram showing eclipse types depending on the relative position of the Moon with respect to Earth. When Earth is in region (A), a total solar eclipse is seen; in (B) an annular eclipse is seen, while in (c) a partial eclipse is observed.

3.5 How the eclipses appear

Partial eclipse: During a partial eclipse there are two points of contact. The first point is the moment of contact between the discs of the Sun and the Moon, which marks the beginning of

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the phenomenon. As the Moon continues along its orbit, an increasing fraction of the solar disk is covered, until the maximum, after which the shadow departs from Earth’s surface and the full disk is visible again. The ‘magnitude’ of an eclipse is the fraction of solar diameter obscured by the Moon (Figure 4). The magnitude can be expressed both in percentage and decimal fraction (60% or 0.60). The term ‘dimming’ refers to the fraction of the solar surface covered by the Moon (Figure 4).

VERY IMPORTANT EYE SAFETY: In a partial eclipse, the Sun is still very bright, so normal

precautions apply to observing the Sun in that case.

Figure 4: Magnitude and dimming of a solar eclipse. The magnitude expresses the fraction of the Sun’s diameter hidden while the dimming is the fraction of the Sun’s surface that is overshadowed.

Annular eclipse: An observer of an annular eclipse will see four moments of contact between the solar and lunar discs. The first contact is the moment when both disks appear to touch. Slowly, in a process that takes about an hour and 30 minutes, the lunar disk appears to fully cover the solar surface; this is the second contact. Then the central or annularity phase, culminating in third contact of the event begins. This phase, can reach about 12 minutes and 30 seconds. The fourth contact refers to the end of the eclipse.

Total Eclipse: A total eclipse also has four contacts. The first contact and the preceding

stage are similar to those in an annular eclipse. But now, before second contact, the observer will see a dramatic change in light. Atmospheric parameters such as temperature and relative humidity are also changed.

If the observer is located in a high place with a good view of the distant landscape, the

moon’s shadow can be seen approaching the western horizon at high speed. At the instant of the

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second contact it produces a diamond ring, a brightness which happens at the point when the sun is almost completely hidden. But before the last portion of the sun disappears, because of the rugged terrain of the edge of the lunar disk, luminous fragments of light, called Baily's beads (Figure 5) can be seen. Then suddenly the outer atmosphere of the sun (the solar corona) appears (Figure 6). In the first few seconds some of the sun’s chromosphere (gases) can be seen as a thin arc of intense red color with bright bumps. These quickly disappear after the advance of the lunar disk (Figure 7).

Figure 5: Composition of images showing the second and third contacts, Baily’s beads, bumps and inner corona in the solar eclipse of July 22, 2009, near the city of Chongqing, China. (Photo J.C. Casado / starryearth.com).

The solar corona (Sun’s outer atmosphere), an intense pearly white colour, shows structures that follow the arrangement of the magnetic field of the Sun. It is not normally visible because it is about 100,000 times less intense than Sun light. In the centre is the lunar disk, like a black hole in the sky. The shape and brightness of the corona depend on where the Sun is in its 11 year cycle. At solar maximum the corona has radial symmetry (Figure 6 right), while at its minimum the coronal feathers are asymmetrical (Figure 6 left).

3.6 Visibility and duration

Total solar eclipses are not as uncommon a phenomenon as might be thought. However,

because the shadow of the Moon is narrow, they are only visible in a relatively narrow band on Earth’s surface, and are observed at a specific point on the Earth's surface, such as a city, only once every 375 years on average. It is therefore necessary to make long journeys to be in the band of totality and to witness the entire event. On average, a total eclipse will last about 3 minutes with the longest lasting up to 7 minutes 30 seconds.

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Figure 6: Left. Picture of the total solar eclipse of August 1, 2008 from Novosibirsk, Russia. Combination of 67 digital images showing long stretches of the corona, bumps, Earthshine and stars. Right. Picture of the total solar eclipse of November 23, 2003 taken on board an aircraft flying over Antarctica. Photos and processing: JC Married / starryearth.com.

Figure 7: Chromosphere and prominences visible in the total solar eclipse of March 29, 2006, photographed near Al Jaghbub in the Libyan desert. Combination of photographs taken at the beginning and end of totality. Picture: J.C. Casado/ starryearth.com.

3.6.1. The Total Solar Eclipse of 2012 After more than a year without total solar eclipses (the last took place on July 11, 2010) the

Moon's shadow will revisit the Earth's surface on 13 November 2012. The tour of the shadow begins in Australia and will then move to the Pacific Ocean. The maximum eclipse occurs in the middle of the Pacific Ocean with a duration of 4 minutes and 2 seconds with the Sun at 68 degrees above the horizon at 22:11 UT.

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Figure 8: Full band (blue lines) of the eclipse of November 13, 2012 as reported by NASA. The green dot indicates where the maximum duration of the eclipse can be seen. The observation point of the expedition will be around the city of Cairns (Queensland state) northeast of Australia.

GLORIA will broadcast the event from three observation points in northwestern Australia (State of Queensland), around the city of Cairns, where the eclipse will have an average duration of about 2 minutes (see Figure 9):

○ G3. In the Coast, in the surroundings of Oak Beach.○ G2. Inside. Highway Rt-81.○ G1. Inside. Town of Mareeba, the coordination and drafting group.

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Figure 9: Observation points of the expedition. The red line indicates the centre of the whole band. The observation points are located on the coast and inland (red dots G1, G2 and G3).

The expedition and retransmission will be coordinated and directed by Dr. Miquel Serra-

Ricart (Astronomer, Institute of Astrophysics of the Canary Islands and Manager of the Teide Observatory).

4. Performing the activity

4.1 Calculation of the thermal response of the atmosphere from atmospheric measurements in a total solar eclipse

An interesting effect that occurs during the course of an eclipse, more remarkable in a total

eclipse, is the decrease of the environmental temperature due to the decrease of the solar radiation or ambient brightness (see Ref. 8 in the reference document). The interesting thing is that the phenomenon does not occur instantaneously when the sun is completely covered (maximum eclipse or second contact), but is an effect that occurs after a time ranging from 2 to 20 minutes.

This time delay depends on many factors, such as the time of day when the eclipse occurs,

the presence of nearby bodies of water such as a lake or ocean, proximity to wooded areas, but it is easily measurable. The time at which the minimum intensity of light occurs, coincident with the maximum Eclipse (second contact) must be noted. Then, the time at which the temperature is a minimum is also recorded. The thermal response of the atmosphere or the atmospheric thermal

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inertia is the time interval between these two minima.

The following exercises (Method 1 and Method 2) can be used to estimate the thermal response of the atmosphere.

Figure 10: Representation of the drop in solar radiation or luminosity (blue) and temperature (yellow) as a function of time produced during the annular eclipse that occurred in October 2005.

4.2. Method 1: Direct shot. Pressure values, radiation intensity and temperature

The live broadcast of the solar eclipse on November 13, 2012 as seen from Australia can be

followed on the GLORIA project website (www.gloria-project.eu). Simultaneously in a parallel web channel, the data measured by a weather station located at point G1 (Mareeba, Australia) of the expedition, can be seen. Students can then record temperature values, solar radiation intensity and air pressure, as if they were in front of the weather station taking the values themselves.

The measurement interval should be periodic and can be established by the student or the

teacher. It is recommended that the interval between two measurements does not exceed 2 minutes and that it reaches 5 seconds when approaching the maximum point of the eclipse (second contact).

Figure 10 shows an example of solar radiation and temperature data taken during the annular

eclipse of 2005, where the measurement interval was variable, with data being taken every 5 minutes initially, then every minute and finally, around the maximum point of the eclipse, every 20 seconds. This measurement sequence was repeated in reverse during the second half of the eclipse. This type of measurement is called "dynamic sampling".

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After taking measurements, there will be a graphical representation of recorded values for each of the variables over time, using any software that allows representation of numeric data (e.g. Excel, OpenOffice, LibreOffice, Origin ...) so that the trend in each of the observables can be seen. Use these graphs to determine the exact time when the minimum occurs for both temperature and solar radiation intensity. By determining the time difference between these values, the value of the "atmospheric thermal inertia" can be obtained.

For an exact calculation of the minimum, curve fitting to the data can be performed. The

teacher will decide whether to instruct students in the curve fitting because it is a topic not included in this unit.

Following the eclipse, a video of the complete recording of data from the weather station will

be available.

4.3 Method 2: Database

During the broadcast of the event, the weather station will periodically save (about every 5 seconds) the values for each of the variables discussed earlier, so that they can be accessed at any time from the GLORIA website (www.gloria-project.eu). Web tools will be provided to enable detailed examination of the data in any time interval requested. For example, time intervals can be selected from different moments during the eclipse. Values of temperature and solar radiation intensity will be plotted on a graph, along with the errors on these values. The interval can be changed (‘zoomed in’), so that more detailed data can be plotted in a particular time interval, to get more accurate determinations of the minimum values for the radiation intensity and temperature.

After selecting the data and making the graphical representation, the "atmospheric thermal

inertia" can be calculated as before.

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REFERENCES

1. SERRA-RICART, M. et al. Eclipses. Tras la sombra de la Luna. Shelios, 2000. Ameno and colorful book without losing rigor, especially dedicated expeditions to observe total solar eclipses.

2. GIL CHICA, F.J. Teoría de eclipses, ocultaciones y tránsitos. Alicante University, Murcia, 1996. Treatise on the theory of eclipses and blockout bodies in general. This is a book that develops in detail the mathematical aspect of these phenomena, which requires advanced knowledge of mathematics.

The wider literature on the subject is in English:3. ESPENAK, F. Fifty Year Canon of Solar Eclipses : 1986-2035. NASA Reference Publication

1178. Sky Publishing Corporation, Cambridge (USA), 1987. Canon or catalog reference made by one of the best specialists, Fred Espenak. Contains data and maps for all solar eclipses from 1986 to 2035 with details and general information for the period 1901-2100.

4. ESPENAK, F. Fifty Year Canon of Lunar Eclipses : 1986-2035. NASA Reference Publication 1216. Sky Publishing Corporation, Cambridge (USA), 1987. Canon containing all data and maps of lunar eclipses between 1986-2035 in detail and general information for the period 1901-2100.

5. MEEUS, J. Elements of solar eclipses 1951-2200. Willmann-Bell, Inc, Richmond (USA). It contains elements of the 570 besselianos eclipses between 1951 and 2200, that let estimate the general and local circumstances thereof. The formulas, with high precision, have been developed by the Bureau des Longitudes in Paris. There is also a version on data with recorded items, but not with a routine use, so it must be programmed.

6. GUILLERMIER, P. y KOUTCHMY, S. Total Eclipses. Springer, 1999. Science, observations, myths and legends about eclipses, especially total solar A great book for anyone who wants to learn more about eclipses and observation.

7. REYNOLDS, M.D. y SWEETSIR, R.A. Observe eclipses. Observe Astronomical League Publications, Washington (USA), 1995. Excellent work of outreach and manual observation, covering all aspects that can span the amateur. It can be purchased through the publisher of U.S. magazine Sky and Telescope, Sky Publishing Corporation.

8. JAY ANDERSON. Environment Canada. Weather, Volume 54, Issue 7,pages 207–215, July 1999. Magazine specializing in the publication of scientific papers focused on the study of the atmosphere and climate.

Finally note the NASA Technical Publication, published some eighteen months before each annular or total eclipse. Collect maps, charts, forecasts and information on general and local circumstances of the eclipse in question. For more information contact Fred Espenak, NASA / GSFC, Code 693, Greenbelt, MD 20771 (USA) or via e-mail: espenak@gsfc.nasa.gov

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